Note: Descriptions are shown in the official language in which they were submitted.
6~'3
The inve~tion rela-tes to a method for pressing salts
in roll presses having rolls driven in unison, the roll surface
being profiled, in particular ridged or honeycombed, where the
sal-t is fed into the nip or gap between the rolls by means of,
for example, screw conveyors.
Certain salts used as fertilisers, in particular po-
tassium salts, are produced during manufacture as material of
relatively small particle size wh.ich is unsuitable, in this form,
for mechanical spreading. These salts for use as fertilisers are
therefore usually pressed in roll presses so as to produce a
granulate with particle size of, for example, between 1 and 4 mm.
Rock salt is also pressed and granulated in a similar manner
when of small particIe size.
When the salts are presse~ by means of rolls having
profiled surfaces a good pick-up of the salt and th~ls a food
filling of the roll surface i.s achieved. Rolls of this type,
like the more generally used smooth-surface rolls, have hitherto
been driven in such a way as to produce shells which do not ex-
ceed thickness of approximately 8 to 10 mm. Hitherto attempts
to increase the throughput have been implemented by increasing
the circumferential velocity of the rolls. However, limits to
the increase in circumferential velocity arise from the fact that
the above cer-tain circumferential velocities it is no longer
possible to achieve satisfactory pressing of the shell.
A criterion for the quality of granulates of salts
used as fertiliser is the so-called abrasive resistance. This
is defined as the amount of salt which i5 abraded from the gra-
nulate during shaking of the granulate with steel spheres within
a predetermined time. The lower the amount of salt abraded the
better is the quality.
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It is an object of the invention to devise a method
by means of which it is possible to produce, with high through-
put/ shells of particularly high strength from which a granulate
with high abrasive resistance can be manufactured.
According to the invention there is provided a method
for pressing salts in a roll press having rolls driven in unison
where the surfaces of the rolls are profiled and the salt is fed
into the nip or gap between the rolls, the method including
a. feeding the material to the press at a temperature
above 12ooc~
b. pressing the material with a specific press force
equal to or greater than 4.5 t/cm width of the roll,
c. driving said rolls at a circumferential velocity
be-tween 0.55 and 0.75 m/sec, and
d. maintaining the surface of the rolls at a tempera-
ture below 80C.
By means of the method according to the invention a
granulate is produced which has an abrasive resistance which is
considerably greater than the abrasive resistance of salts press-
ed by known methods. The abrasive resistance is improved by amagnitude of 30-50~.
~ urprisingly, it was found that, in spite of the rela-
tively low circumferential velocity, a high throughput can be
achieved and, in particular, it is possible in the method accord-
ing to the invention to increase the thickness of the shell con-
sidera~ly. Shell thicknesses greater than 14 mm can be achieved
by the method according to the invention. Shell thicknesses of
16 mm have been achieved with a similar abrasive resistance of
the granulate. When the thickness of the shell is increased it
is necessary to adjust the specific press force. For example,
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~:309G~
the shell thickness of 16 mm was achieved for potassium salts
by means of a speciflc press force of about 5.5 t/cm. A con-
siderable improvement in the circulation ratio of feed material
to shell is also achieved in the method according to the inven-
-tion. For example, a ratio of shell to feed material greater
than 90% was achieved, that is at least 90% of the feed of sma~ll
particle size was pressed to form
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a shell capable of being worked further.
The shell was then broken by using an impact pulveriser. The
granulate was then separated into three fraction3 by sieving.
The granulatewith a predetermined particle size, For example 1-4 mm,
was removed as granules of usable size. Granulate with a particle
size less than 1 mm. - undersiz0 - was returned to the fine feed
material for the press and pressed againO Granulate with particle
size greater than 4 mm was milled again.
A ratio of undersize to usable granules of about 1:1 i5 achieved
1~ in the method according to the invention. The circulation arnount
is thus reduced to about half the total flow of material. In ~nown
smooth roll presses the material circulated is yreater than 30~0
of the usable granules produced.
Thus a decrease in the specific electric power consumption is
qchieve~J by comparison with smooth roll presse3. Calculated on
the total plant, it is possible to count on a halving of the
specific electrical power consumption, that is the amount of
electrical power consumed for unit weight of usable material.
Owing to the relatively low circumferential velocity together with
the profiling of the roll surfaces a particularly favourable
de-aeration of the material is also achieved so that it is possible
to produce a shell substantially free from interruptions.
Because of the temperature of the roll surFaces~ wllich is below
BûC, in particular within the range 40C to 70C, melting of the
salt on the surface of the roll is avoided. The shell has a hard
polished surface. The surface temperature which it is possible
to achieve on the roll depends essentially on the temperature of
the cooling water available and on the temperature of the
surroundings. Attempts should be made to secure as low a temperature
as possible, preferably within the range from 40 to 60C.
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The following are used for the method:
1. A roll press with press rolls arranged horizontally against
; one another.
1.1 Roll diameter 1000 mm.
1.2 Working breadth of the rolls 'L250 mm.
; 1.3 Surface honeycombed with cushion-type depressions wlth
1.3.1 maximum depth 5 mm.
1.3.2 lateral displacement of the honeycomb by 50~0 in
successive rows.
1.3.3 displacement of the honeycombed depressions ~y 50
in the circumferential direction on the rolls
applied against each other.
1.4 Cooling of the rolls by means of cooling medium chan~els
arranged below the surface of the rolls
1.5 Drive
1.5.1 installed operating,power 630 KW
1.5.2 r.p.rn. of the rolls 12 r.p.m., corresponding to a
circumferential velocity of 0.6 m/sec
1.5.3 synchronised drive of th~ roll pair.
1.6 Feed by rneans of filling screws, 5-fold.
2. Feed material loose potassium salts
2.1 Particle si~e 60 (60 potassium salt)
2.2 salt temperature 140C.
3. ûperation
3.1 mean thickness of shell 14 mm.
3.2 specific press force 4.5 t/cm
3.2 throughput
~ 3.3.1 gross 80 t/h
; 3.3.2 shell production 75 t/h
3.3.3 yield of granulate particle size 1-4 mm 45 t/h
3.3.4 Abrasive resistance high.
3,4 Power consunnption of the press 425 KW.
4. When the mean thickness of the shell is increased to 16 mrn,
with a proportional increase in power consumption, a proportional
increase in the throughput data of less than 3.3 is achieved with
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the ~ame granule quality.
~y way of further axample, the method according to the~invention
is illustrated in the accompanying flow diagram. The ~alt,
for ex~mple a loose potassium salt with a particle size less
S than 0.2 to 0.3 mm., is~ as shown by the arrow 2, led to a
bunker 4 from which it is fed ~hroughl a conduit 6 to a roll
press 8 provided with two rolls rotating in opposite directions
about parallel horizontal axes. The material is fed into the
nip or gap between the rolls by means of screw conveyors, and
in particular by means of a multip~icity of screw conveyors
arranged side by side in the axial direction of the rolls.
The rolls are profiled on their surfaces, in particular ridged
or honeycombed. The rolls are cooled and for this purpose
tlle roll pres~ provided with ~n :Inlet 10 and an outlet
12 for cooling water.
The shell pressed in the roll press ~ i3 conveyed to an irnp~ct
pulveriser 16 by means of a conveyor device 14. The pulvsrised
matarial at lB is conveyed to a threefold sieve 20~where material
of large particle sizeJ greater than 4 mm. is first removed by
sieving. This material is returned to ths material fIow before
the impact pulveriser :L6 and is again pulverised.
The material with particle size from 1 to 4 mm. is the usable
material.
The fines, with particle size less than 1 mm. which are sieYed
off are returned to the feed bunker 4 by means of the conveyor
davice 22.
The material pressed in the roll press contains a certain
~nount of unpressed fines. Material wi-th a particle si~e
le~s than 1 mm. is sieved off in a sieving device 24 and is
returnedJ together with khe fines sieved off in the sieving
device 20, to the feed bunker 4 by means of the conveyor device
22.
